A window or a door comprises a stationary frame and two leaf frames mounted on this frame so that they pivot about mutually parallel pivot axis. A first leaf frame of these two leaf frames is intended for frequent pivoting movement about a first pivot axis betwen an open position and a closed position. A second leaf frame is intended for less frequent pivoting movement about a second pivot axis between a closed position and an open position. Both leaf frames comprise, remote from the associated pivot axis a leaf frame member the first leaf frame having a first leaf frame member, and the second leaf frame having a second leaf frame member. The first leaf frame member and the second leaf frame member are in the closed position of the associated leaf frames directly opposite each other, i.e. without the interposition of an intermediate member which is rigid with the outer frame. The second leaf frame is in the region of the second leaf frame member lockable by second locking means on transverse members of the frame which extend substantially at right-angles to the first and second leaf frame members. The first leaf frame is lockable in the region of the first leaf frame member by first locking means at the second leaf frame member. The locking system comprises, mounted on the second leaf frame member between the two transverse members a second gear unit. A second drive rod assembly is mounted on and parallel with the second leaf frame member and has two second drive rod elements adapted for movement in opposition to each other and parallel with the second leaf frame member as a result of actuation of the second gear unit. Second drive rod end bolts are provided on both second drive rod elements for engagement in corresponding second drive rod end bolt housings on the two transverse members. The said locking system furthermore comprises a first gear unit mounted on the first leaf frame member between the two transverse members. By reason of the first gear unit, transverse latch means are controlled. These transverse latch means have at least one transverse latch crossing a leaf frame gap such as to engage transverse latch housing means which can be provided on the second leaf frame member. The first gear unit further controls transverse bolt means crossing the leaf frame gap such as to engage bolt housing means which can be provided on the second leaf frame member.